Metal alloy



Aug. 3@, 1949.

Filed Dec. 23, 1946 V. T. MALCOLM METAL ALLOY 2 Sheets-Sheet l mg. 3%. 1949. v. T. M-ALCOLM METAL ALLOY 2 Sheets-Sheet 2 Filed Dec. 23, 1946 Creep Rofe- Percenf per 1000 Hours 1 Z I IN V EN TOR.

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Patented Aug. 30, 1949 METAL ALLOY Vincent T. Malcolm, Indian Orchard, Mass, assignor to The Chapman Valve Manufacturing 00., Indian Orchard. Mass., a corporation of Massachusetts Application December 23, 1946, Serial No. 717,855

3 Claims.

within the range of from 600 to 4000 pounds per square inch and at temperature between 900 and 1100 degrees Fahrenheit has certain deteriorating influences and reactions on prior art carbon molybdenum steels which according to this invention are obviated as will hereinafter appear from the following description.

With the foregoing and various other novel features and advantages and other objects of my invention as will become more apparent as the description proceeds, the invention consists in certain novel features of construction and in the combination and arrangement of parts as will be hereinafter more particularly pointed out in the claims hereunto annexed and more fully described and referred to in connection with the accompanying drawings wherein:

Figs. 1 to 6 inclusive are reproductions of photomicrographs of steels at 100 and 500 magnifications respectively to explain the novel features of the invention; and

Fig. '7 is a reproduction of a chart showing the creep rate of steels to explain the novel features of the invention.

Referring now to the drawings more in detail, the invention will be fully described.

What of late has become known as graphitization has been the cause of considerable concern in the operation of major steam generating stations using steel pipe, valves, fittings and other apparatus at high pressures and temperatures within the range of 900-1100 F. This is due to the failure by fracture or brittleness of certain specified carbon molybdenum steels corresponding to a chemical composition of:

Carbon .05-.35 Manganese .70 max. Phosphorus .05 max. Sulphur .06 max. Silicon .60 max. Molybdenum .40-.60

These failures have been very definitely caused by excessive local "graphitization. Examinations of many steam lines throughout the country have revealed instances of local graphitization to the extent that this type of deterioration has become of grave concern to the entire power industry.

Carbon molybdenum steel of the above composition has heretofore been recognized in the steam industry as a valuable material to withstand high pressures and high temperatures in central stations due to its strength and toughness, and especially due to its excellent creep resistance under elevated temperatures within the range of 900-1100 F. However, due to the excessive local graphitization which has taken place with the consequent danger of premature failure, newer types of steel having more stable characteristics are necessary.

Therefore, the principal object of this invention is to provide a steel which is free from the deteriorating effect of graphitization caused by instability of the carbide structure which at the same time has higher physical properties and better creep resistance within the temperature range of 900-1100 F. than the previously known steel. A complete study has been necessary in order to determine carbide forming elements which would maintain their structural stability within the 900-1100" temperature range.

It has long been recognized that the FeaC type of carbide found in the prior steel is not a stable phase, and that if given a sufficient time at a temperature within the 900-l F. range it will decompose with the formation of graphite. Evidence has been found of structural instability of the carbon molybdenum steels referred to within this temperature range and as a result of these findings research has been carried out which resulted in the more stable type of steel hereof.

In the carbon molybdenum steel, previously referredto, the carbides cannot be entirely considered to be of the nature of FeaC, which is known to be metastable with respect to graphite, but may also contain molybdenum in solution in the ferrite; and in a given steel of this composition different types of carbides may be expected to exist at different temperatures or after different times at a constant temperature and more than one type may coexist under certain conditions of time, temperature and stress.

Aluminum is nearly always added to carbon molybdenum steels as a deoxidizing agent and reacts above the critical point in such a manner that the carbides form a homogeneous, probably colloidal solution, which persists until the critical point is reached. And as the gamma iron goes over to the alpha state in which the carbides are only slightly soluble, the precipitated carbides itization in carbon molybdenum steel but has w great bearing on the production of sound steel castings free from hot tears etc., While metal- H or acid soluble aluminum induces figraphiti zation without any advantage to the steelmal ing practice.

It is, therefore, necessary in the manufacture of cast steels to add a certain :quantity of aluminum in order to obtain good castings but the aluminum addition must be so adjusted that the residual quantity must be under .05 and remain mostly in the oxiiie zform, and may b'e ibrought about by'the judicious addition of carbide dorming alloys which increase the structural stability of this type of s'teel.

Chromium, vanadium :or titanium, each rat which when added in feertain "quantities to carbon molybdenum Ste'el, are :known to :produce a more stable carbide when the steel is :used within the temperature trange of 900-11-00" and :due to this increased stability twill iimprove theeresistiancebf the stee1itofigraphitizationbutinrordehto obtain -the maximum resistance to lgraphitiza'tion :it is necessar Eto :add cei taiin amounts of chromium, vanadium iand titanium in the iproper combination to carbon molybdenum--stee1and-at the sam'e time exercise the a udicio'us us'e or sin- "niinum as afdeoxidiz'ing agent.

"Therefor raccdrding to Zthis invention *1 proviile a :new steel iha'ving the following chemica'l' composition:

The previous heat treatments used as a standard in the treatment of carbon molybdenum steel was to normalize at 1650 F., followed by a temperating treatment at 1200 F. and slowly cooled.

The physical properties at room temperature of the regular carbon molybdenum steel are specifiedzasifollows:

Tensile strength, p. s. i 70,000 Yield strength, p. s. i 45,000 'Elongation in 2" percent 22 ERednction of area, percent 35 .The physical ,properties of the steel of my in- :veniiion .are as follows:

' Tensiles'trength, -,p. s. i 80,000 Yield strength, p. s. i 50,000 Elongation in 2" percent 22 Reduction of area, percent 35 The creep limit of thelcarbon molybdenum steel at 1000.-F.- causing .1% creep in 100,000 hours is 8000 ,pounds ,per-square finch, whereas in thasteel of my invention the creep limit at 1000" Facaus- .ing .1;% -creep 100,000 *hours is 12,900 pounds ,pensquare inch.

Theehart-of Fig. I? shows the=diiference =int'he creep limits of the two .steels. .In said .=char.t C. .:M. V. .means ;Chapman carbon-molyb- .denum-chromium-yanadiumatitanium steel :of .the invention and C. Mo. :means the prior art :regular carbon molybdenum steel.

il estsrforresistance tog-raphitizationihave been carried'out over long periodsiof time and the ,phovtomicrog-raphsof (Figs. 1 .to -6 inclusive show the conditions that exist .in .the:several steels v after test at temperature. The photomicrographs are as .follows:

.Figs. .-1.and (2 are ,photomicrographs of WG-1 .type,.-A. S..M. .A-217, (atl00 and 500 magnifications respectively and discloses the existence 4J1 .nodulargraphiteafter 3000 hours atl950 :It

'It is also of importance that special Icar'ebe giventomethods 'o'f heattreatment ofs't'e'ls, and a greatipa rt of theresearc'hoh the'stels investigated was 'dete'rminedfbyxeray diffraction, "metallograp'hy, lmicro-"radio'graphy and with .sens'itive "dilatometers. Experimental work followed by practical applications provided s'ufiicie'nt data. so that previous heat treatments of the steel c'ollldbe' modified with fdefinite;improvemeiit and the ideal structure Tor-"stability obtained.

I "have discovered that :a "heat "treatment consisting ofthe "following steps win .rie'sult'i'n .a "Steel having'the 'desiredideal structure:

Homogenize'at 1750 F1 6 houts Normalize at 1650 per 'htour -'6f thickhisss of-heaviest section of metal Iemper at 1300 1325 per h'our (if "thick- In the tempering -operation, ithedeoolingishould be very slow between the l300 :to 4000 range and may be accelerated-=from the lower'tempera ture.

xvanadium according to this :invention. .No graphitization is disclosed in this .typeofsteel aiter 40,000 hours {at 1025" In accordance with my invention "and ,heat treatment, the steel .does not show ,anytendency towards graphitizationaiter l0-,000hours,at 1025 FL, whereas .the originalrcarbon. molybdenumrand modified ,canbonanolybdenumado-showthis tendency .evemin a shorter time.

Another :important rfeature of :my invention .the. control-ofggrain size which may rbeskept-suniform .from iheat to :heat while 'at the 'same time the steel hasexcellentzweldingecharacteristicsand is not brittle when properly preheated and stress relieved.

"Ihe proper'preheat -is -600 F. and "during the welding operation this *preheat 'should never *be allowed to'drop'beiow-550 F. 'It-is'also desired .70 that'nthe steel bewarmednpforseveral hour-sat this temperature before starting "the "welding operation.

.il heistress'relief; or;post:heat =treatment,sshould bewarried -:out at 1 a temperature -'of 1e00 wand .76 *newergreater than:1-325 F. andzthe'-.-timezat temperature should be A" per hour of thickness of weld after which it must be slowly cooled through the range of 1300 to 1000 F. and from the lower temperature cooling may be accelerated.

The invention may be embodied in other specific forms without departing from the essential characteristics thereof. Hence, the present embodiments are therefore to be considered in all respects merely as being illustrative and not as being restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all modifications and variations as fall within the meaning and purview and range of equivalency of the appended claims are therefore intended to be embraced therein.

What it is desired to claim and secure by Letters Patent of the United States is:

1. A machinable readily weldable carbon alloy steel consisting of in combination, between 05% and 35% carbon, no more than 370% manganese, about .60% chromium, about 55% molybdenum, about .12% vanadium, and about .03% titanium, with the balance consisting of iron and incidental impurities.

2. An alloy steel having the necessary physical properties ior utilization at high pressure and at high temperatures approximating 1000 degrees Fahrenheit consisting of, between .05% and carbon, about .70% manganese, about .60% chromium, about .55% molybdenum, about .12% vanadium, and about 03% titanium, with the balance consisting of iron and incidental impurities.

3. An alloy steel having a chemical composition for use with high temperature steam and adapted to resist local graphitization consisting of, between .05% and 35% carbon, about .70% manganese, about .60% chromium, about molybdenum, about 12% vanadium, and about .03% titanium, with the balance consisting of iron and incidental impurities.

VINCENT T. MALCOLM.

REFERENCES CITED UNITED STATES PATENTS- Name Date Strauss Aug. 4, 1942 Number 

